At present, more plant protein feedstuffs are used in aquatic feeds for the shortage and the increasing price of fish meal. The deficiencies of some essential amino acids (EAA) in plant protein feedstuffs lead to the addition of some EAA to balance diet composition. However, the effect of supplementing crystalline EAA was not positive for some fishes. As a result, the present experiment was conducted to investigate the effect of crystalline or microcapsuled lysine supplementation into practical diets on growth performance, and the total plasma free amino acids content of grass carp Ctenopharyngodon idellus. Four diets were prepared, which were high soybean meal（SM）diet with 23% SM, low SM diet with 15% SM, low SM diet added with crystalline or microcapsuled lysine, which had the same lysine content as high SM diet respectively. Grass carp (49.0±2.0) g were fed the four diets with triplicate for 8 weeks. Growth rates(GR) of the four groups were 279.3%, 239.7%, 245.6%, 277.5%, feed coefficient(FC) were 1.62, 1.88, 1.85, 1.64, respectively. Compared with low SM group, adding crystalline lysine in diet had no significant effects on growth performance, but the growth rate was improved by 15.8% and FC decreased by 0.24 (P<0.05) by adding microcapsuled lysine in diet. Fish fed on low SM added with microcapsuled lysine had the same GR and FC as those of high SM group(P>0.05). Plasma total free amino acids(TFAA) concentration at 0, 1, 2, 3, 4, 5 h after feeding were determined, and the highest concentration of TFAA occurred at 3 h after feeding for high SM diet, low SM diet added without or with microcapsuled lysine, but absorbing peak occurred at 2 h after feeding for low SM diet added with crystalline lysine, which means the absorbing peak was advanced. Leaching loss of crystalline and microcapsuled lysine were 69.82% and 19.81%, which indicated leaching loss of crystalline lysine was significantly decreased by microcapsuling. Results above showed that grass carp fed with diet supplemented with microcapsuled lysine had a slower absorbing peak of serum TFAA than that fed with crystalline lysine. Microcapsuled lysine had a lower leaching loss in water than crystalline lysine. Growth rate of grass carp could be improved, FC decreased by adding microcapsuled lysine in low SM practical diet, but was not affected by crystalline lysine.